With use of the Doppler ultrasonic flowmeter catheter, phasic aortocoronary bypass graft blood velocity was measured in 16 conscious subjects during ventricular arrhythmias. Ventricular extrasystoles reduced peak systolic and diastolic graft blood velocities by 20 to 80 percent, generally in relation to their respective coupling intervals. When extrasystoles appeared in closely coupled salvos diastolic bypass blood velocity virtually ceased. Nineteen episodes of ventricular tachycardia produced an average 50 percent decrease in peak graft blood velocity (control mean +/- 1 standard deviation blood velocity 28 +/- 11 cm/sec; value during ventricular tachycardia 14 +/- 8 cm/sec, P less than 0.001). An "overshoot" of peak blood velocity was observed after ventricular extrasystoles and tachycardia. All such changes in aortocoronary bypass blood velocity related to tachyarrhythmia were more prominent during the systolic fraction of flow. It is concluded that (1) ventricular arrhythmias adversely influence aortocoronary bypass graft function, and (2) this finding supports an aggressive approach to the treatment of these arrhythmias in subjects with bypass grafts.